The logic of restricting the search for life to the habitable zone is clear: we have a single example of life, and it arose right here on Earth. Our planet lies in a low-eccentricity orbit around a hydrostatically stable, main sequence, Population I star, at a distance where water can remain liquid for billions of years.

There’s little point in pointing Kepler towards stars like Eta Carinae, as interesting as it might be in itself, because it is of relatively short lifespan and is “prone to violent outbursts” that will have obliterated any planets in its orbit. Nor is there much point in looking towards stars like Vega, whose spectral classification leads scientists to believe it is particularly poor in metallic elements, which are important in planet formation.

Moreover, high-metallicity may indicate that the star formed in an accretion disk rich in elements heavier than iron, which cannot easily form in stellar nucleosynthesis. Simple life-like chemicals like those postulated for the “primordial soup” may have been composed entirely of carbon, hydrogen, nitrogen, oxygen and a smattering of phosphorous. But many of the elements essential for life on Earth are heavier than iron, so it would be helpful if our candidate stars were formed from the debris of supernovae.

Despite these restrictions, we are fortunate enough to have a “few dozen” sun-like stars within 30 light years of our own, and perhaps half of these will have rocky planets in their orbit, according to Dr Alan Boss of the Carnegie Institution for Science.

So Kepler is presumably looking in the right direction. But, having identified planets where life might arise, what might we expect to find if we could visit?

Again, since we only have one example to work with, we may as well take a look around on Earth. The IUCN Red List gives the following numbers of described species in the major divisions of life:

Invertebrates: 1,232,384

Plants: 298,506

Vertebrates: 61,259

Lichens, fungi and brown algae: 50,040

Microbes: 100,000 – 1,000,000

The microbial species are still a subject of controversy: a recent estimate by scientists at Cornell puts the number at 150,000 (thanks, Tim), an order of magnitude lower than previously believed.

The relative number of species is to say nothing of their relative abundance, of course. So it’s instructive to note that, phylogenetically, we have more in common with a dromedary, or a stegosaurus, or a coelacanth, or even a sea squirt, than any of the myriad creatures you might scoop up in a handful of soil.

This makes the panoply of “aliens” common to space opera seem depressingly unimaginative. Vulcans, Klingons, Romulans, Cardassians, Wookies, Ewoks, and whatever the hell Yoda is supposed to be are all virtually indistinguishable from humans apart from the addition of a prosthetic nose or a skin complaint.

A couple of early Star Trek episodes at least nodded towards the possibilities, with parasitic flying omlettes that drive the crew of the Enterprise to the brink of insanity, and the Horta: a corrosive subterranean silicate blob which clashes with miners before it is subdued, etching its pleas for mercy in the floor of a cave.

The Horta shows that silicon enjoyed some popularity as a theoretical basis for alien life in past years, since lies in the same group as Carbon on the periodic table and has four valences. Unfortunately, however, there are no known conditions in which Si forms the long-chain polymers common to organic chemistry, and thus life.

In science fiction proper, meanwhile, Stanislaw Lem’s Solaris presents us with an entire planet that seems to be a single, sentient organism, presaging Lovelock’s Gaia hypothesis a decade ahead of its publication.

Given the extraordinary variety of life on Earth, the probability of finding bipedal aliens of about our stature are vanishingly small. The question of size was memorably examined by Douglas Adams, when an entire alien invasion fleet is devoured by a small dog after a “terrible miscalculation of scale”.

At the other extreme, discounting for the moment planet-sized organisms, we might reasonably expect to meet gigantic creatures. Elephants remind us we are by no means the largest terrestrial organisms, to say nothing of the blue whale.

Which brings us neatly to the fourth dimension. Breaking down the 4.5 billion years of Earth’s history, we have:

1 billion years as a lifeless rock

2.5 billion years of single-celled organisms

250 million years of multi-cellular organisms

50 million years of cephalopods and arthropods

200 million years of fish

140 million years of reptiles

65 million years of mammals, including 10 million years of large primates

100,000 years of anatomically modern humans

It is disheartening to look at this list, and think that any aliens capable of visiting Earth to make contact may have already been and gone, with only Eusthenopteron around to tell the tale.

And yet, according to some researchers, the best place to look for “alien” life may be right here on Earth after all. Paul Davies of Arizona State University in Tempe postulates “shadow life” in an article in Astrobiology.

All life on Earth is generally understood to be monophyletic. This does not preclude the possibility of life having arisen multiple times, only to be obliterated time and again by the late heavy bombardment, or simply out competed by our ancestors.

Discerning shadow life from the ordinary variety will be tricky, but one way would be to look for unusual uses of chemicals. Ordinary life almost exclusively uses right-handed isomers of sugars, and left-handed isomers of amino acids. Detecting either of the reverse might indicate life of a different ancestry, though recent studies suggest water may have a role in explaining the enantiomeric excess: asteriods contain 15% more left-handed amino acids. Organisms that use arsenic in place of phosphorous for energy transport (adenosine triarcenate?) would be weirder still. Arsenic’s very toxicity in ordinary life is due to its chemical similarity to phosphorous.